This invention relates to the moving-target radar art and has particular relationship to radar for detecting objects moving relatively slowly such as the movement of intruders into a protected area. In the use of such radar a signal having a frequency typically of the order of several hundred megacycles is transmitted into the protected area. Objects moving in the area reflect a resulting signal dependent on the movement. In effect the transmitted carrier signal is reflection phase modulated by the parameter of doppler frequency dependent on the movement of the objects. This resulting signal is received and processed to derive a signal of doppler frequency which serves to indicate the intrusion. Typically the doppler signal has a frequency of 10 cycles per second or less.
The prior art is typified by the teachings of the articles by H. P. Kalmus, Direction Sensitive Doppler Device in the June 1955 Proceedings of the IRE beginning page 698 and Doppler Wave Recognition With High Clutter Rejection Document AD 652,878 TR-1349 of Harry Diamond Laboratories, Washington, D.C., Apr. 1967. This prior art includes facilities for determining the direction of movement of a target but has the disadvantage that it is characterized by microphonics. Other typical prior art includes a balanced processor for determining the direction of movement of a target but has the disadvantage that the channels must be closely matched.
It is an object of this invention to overcome these disadvantages of the prior art and to provide a moving target radar capable of detecting a moving target in the presence of substantial clutter and determining the direction of movement of the target without the microphonics and/or the demand for matched channels of the prior art. It is another object of this invention to provide such a radar particularly suitable for the detection of objects or bodies intruding into a protected region. A further object of this invention is to provide a variable-bandwidth bandpass filter of relatively simple structure and relatively low cost. A still further object of this invention is to provide a precise multiplier for multiplying electrical parameters.